Dampening systems are used on lithographic printing presses to apply dampening fluid to a plate cylinder on the press. The plate cylinder has wrapped around it a chemically treated plate with hydrophilic (water-loving) areas and oleophilic (oil-loving) areas in its outside surface. These hydrophilic and oleophilic areas are arranged in a pattern on the printing plate to produce the desired image on paper. The oleophilic areas attract the oil-based ink and repel the water-based dampening fluid, while the hydrophilic areas attract the dampening fluid and repel the ink.
The dampening system applies dampening fluid either directly to the plate cylinder by way of a separate dampening roller or indirectly to the plate cylinder by way of the inking form rollers. The form rollers apply a thin layer of ink and dampening fluid to the respective areas of the printing plate on the plate cylinder. The proper proportions of ink and dampening fluid as applied to the plate cylinder (referred to as the ink-water balance) must be maintained for the proper application of the ink to the paper. If there is too much dampening fluid relative to the ink, the ink on the paper will lose color and fade. If there is too little dampening fluid, ink will appear on the paper in non-print areas.
Prior art dampening systems suffer from several disadvantages. One such disadvantage is the use of isopropyl alcohol as a wetting agent in the dampening fluid. The dampening system of U.S. Pat. No. 3,168,037 requires a wetting agent such as alcohol to properly mix the ink and water together.
The use of alcohol was initially hailed by the printing industry as an improvement over the prior art, which used a cloth (specifically molleton) covered ductor roller and cloth covered dampening form rollers. Unfortunately, the cloth covers on the rollers required frequent changes, resulting in down time of the press. Furthermore, the dampening system produced variations in ink color throughout a print run.
The use of alcohol in the dampening system of the '037 patent eliminated the need for cloth covers and ducting rollers, thereby increasing the operating time of a press and reducing the maintenance requirements. Isopropyl alcohol, however, is dangerous to work with, being highly flammable and carcinogenic. Alcohol evaporates easily, filling the press room with fumes that are breathed by personnel. Alcohol evaporation can be reduced by the installation of a refrigeration system on the press to cool the dampening fluid. Alternatively, a high capacity ventilation system can be installed in the press room to quickly remove alcohol vapors. In fact, some jurisdictions require such ventilation systems for safety reasons. Either alternative, refrigeration or ventilation, is expensive. In addition, alcohol is expensive to buy thereby increasing the operating costs of a print shop.
Alcohol substitutes have been developed, but none are entirely satisfactory. Such substitutes leave residues on the rollers, requiring the press to be stopped periodically for roller deglazing In addition, alcohol substitutes are difficult to use with respect to achieving the proper ink-water balance.
Another disadvantage of prior art dampening systems is that frequent manual adjustments (mechanical and electrical) are required during the operation of the press to change the amount of dampening fluid that is applied or metered into the system Such adjustments are required because the conditions of the press change during operation, thereby affecting the ink-water balance. When a press is first started in the morning, all of its components are at room temperature. As the press operates over a period of time however, the components heat up. This reduces the viscosity of the ink, thereby allowing more ink to flow Consequently, more dampening fluid is required The operator is required to monitor and continuously adjust the metering of the dampening fluid. However, on most high speed presses, the operator is kept busy enough monitoring the other functions of the press.
Another disadvantage relates to the removal of hickeys from the plate cylinder All printing presses suffer from problems caused by hickeys. Hickeys are small particles of matter, such as paper, dust, dried ink, etc., that adhere to the plate cylinder and the blanket cylinder. Hickeys adhere to the plate cylinder, causing imperfections in the application of ink to the printed paper. The prior art uses such techniques as manual cleaning with a scraper blade or cleaning with an operator's thumbnail Both of these techniques, which are performed during the operation of the press, are highly dangerous, and run the risk of both operator injury and press damage Alternatively, the press is frequently stopped and the plate cylinder is washed down resulting in down time of the press.
The prior art has used dampening systems to clean hickeys off of plate cylinders. Domotor, U.S. Pat. No. 3,467,008 teaches the use of either an inking or a dampening roller to clean hickeys off of the plate cylinder. The roller contacts the plate cylinder and is rotated at different speeds than the plate cylinder MacPhee, U.S. Pat. No. 4,724,764 teaches the use of a dampening roller and an ink receptive roller contacting the dampening roller to remove hickeys from the plate cylinder. The dampening roller contacts the plate cylinder and both the dampening roller and the ink receptive roller are rotated at differential speeds with respect to the plate cylinder.
The problem with the Domotor and the MacPhee systems is that once the hickeys are cleaned off of the plate cylinder, they are either mixed in with the press inking system or accumulate on an ink roller, wherein the hickeys can be reapplied to the plate cylinder. Furthermore, the form rollers that rotate at a differential speed with respect to the plate cylinders are in constant contact with the plate cylinder during the operation of the press. This produces unnecessary wear on the printing plate that is on the plate cylinder.